1. Field of the Invention
The present invention relates to a semiconductor device that is formed by face-down bonding a semiconductor chip on a mount substrate or board, and relates to a method of producing bumps for a semiconductor chip.
2. Description of Related Art
FIG. 15 is a front view showing a conventional semiconductor device. In the drawing, reference numeral 1 denotes a mount substrate; 2 denotes a first semiconductor chip die-bonded on the mount substrate 1; 3 denotes a bonding wire electrically connecting the mount substrate 1 and the first semiconductor chip 2; 4 denotes a second semiconductor chip that is flip-chip-bonded on the first semiconductor chip 2; and 5 denotes a plurality of bumps that are prepared previously on the second semiconductor chip 4. All of bumps 5 are an electrically conductive material such as solder or gold, the bumps are of uniform height, and the bumps are connected with the pads of the first semiconductor chip 2.
FIGS. 16A-16G are explanatory diagrams of a method of producing the bumps 5. As shown in FIG. 16A, the peripheral portion of pad 6, formed of aluminum or the like, is covered with passivation film 7 in the initial step of the wafer processing. When producing the bumps 5, first, as shown in FIG. 16B, under-bump metal 8 is formed over the pad 6 and the passivation film 7 is formed by sputtering. Then, as shown in FIG. 16C, production mask 9 is placed over the under-bump metal 8, and the opening 9a of the mask is positioned above the pad 6. After that, as shown in FIG. 16D, the opening 9a is filled with solder 10. Subsequently, as shown in FIG. 16E, the production mask 9 is removed, and as shown in FIG. 16F, the under-bump metal 8, except its portion located under the solder 10, is removed by etching. Finally, as shown in FIG. 16G, the solder 10 is heated and melted through use of a reflow furnace, to thereby form bump 10A.
The conventional semiconductor device is arranged as mentioned above. That is, a single second semiconductor chip 4 provided with the bumps 5 of uniform height is purely flip-chip-bonded only on the top surface of the first semiconductor chip 2. As a result, there is the problem that high-density wiring is difficult in the conventional semiconductor device. Moreover, the first semiconductor chip 2 is electrically connected with the mount substrate 1 by use of the bonding wire 3. As a result, there is the problem that reduction of wiring capacitance caused by routing the bonding wire 3 is extremely difficult when the semiconductor chips 2 and 4 are high frequency semiconductor chips.
The present invention has been accomplished to solve the above-mentioned problems. An object of the present invention is to provide a semiconductor device in which a plurality of semiconductor chips can be mounted on a substrate at high packing densities without using any bonding wire or by using a reduced amount of bonding wire, and is to provide a producing method of bumps for the semiconductor chip for producing the semiconductor device.
According to a first aspect of the present invention, there is provided a semiconductor device in which a semiconductor chip having a plurality of bumps of different heights is face-down bonded on at least either of a mount substrate and another semiconductor chip.
Therefore, the electrical connections between a semiconductor chip and a mount substrate and between the semiconductor chip and the other semiconductor chip can be performed by use of only bumps. As a result, a semiconductor chip can be mounted over the other semiconductor chip mounted on a substrate without using any bonding wire; semiconductor chips can be mounted at high packing densities without using it; and simultaneously the wiring capacitance can be reduced. Additionally, because any bonding wire is not used at all, the mounting method is extremely useful particularly when the semiconductor chips are high frequency chips.
Moreover, according to a second aspect of the present invention, there is provided a producing method of bumps for a semiconductor chip by comprising the steps of: providing a production mask on the semiconductor chip in succession such that the opening of the production mask is positioned on the pad on which a higher bump is to be formed; filling the opening of the production mask with electrically conductive material every time the production mask is provided; and removing all of the production masks.
Therefore, according to the present invention, two or more types of bumps in height can be easily formed. In addition, conventional bumps of uniform height can be changed into bumps of different heights.